Imaging devices for cameras commonly employ a focusing lens whose image field is folded to produce a focused image in as small a volume as possible. The folding elements employ mirrors which are usually oriented at angles of 45.degree. with the optical axis to fold the image field along directions which are transverse to the optical axis.
Projection display devices are widely used for enlarging small image sources by throwing them upon a viewing screen. Often the image source is some type of microform document, e.g. microfilm strip or a microfiche sheet containing graphic information of extremely small size, which must be enlarged by projection in order to be readable. or the image source may be a miniature cathode ray tube with such a small screen that reading of a detailed image is difficult, and optical projection is needed for the purpose of enlargement. In other instances, the screen of the CRT may be large enough for direct viewing by a small group, but there may be a need to project an enlarged image upon a theater-size screen for display to a large assembly or for other purposes.
Rear projection screens are translucent and have a light-dispersing frosting or coating. The image is seen from the front of the screen, but projected from behind the screen, from the interior of an opaque housing. Such projection devices are subject to degradation of the projected image by ambient light, and consequently they cannot be used easily under ordinary lighting conditions.
Ambient light striking the front surface of a rear projection screen is divided into two components, one reflected from and the other transmitted through the translucent screen. Until now the image degradation effect of the transmitted component has not been dealt with successfully in the design of rear projection equipment. This transmitted component passes through the screen into the interior of the projector housing, and there it strikes a mirror behind the screen, which performs an image beam folding function. In previous devices, this mirror was inclined upwardly in order that an image could be projected from a microform document or other image source located below the projection screen. We have observed that most sources of ambient light are located above the level of the projection equipment. For example, both sunlight and the light from interior room fixtures usually originates above the level of the desk or table upon which the projection display device is employed. As a result, such light is downwardly incident upon the projection screen, and the transmitted component of it tends to strike the upwardly inclined mirror inside the housing. It is then reflected back toward the display screen along an upward path which is generally similar to the path of the image beam.
Prior art rear projection display devices give poor results in brightly lit environments, and it is generally considered necessary to use such equipment in semi-darkness if a high quality image is desired. It appears to me that the principal cause of such image degradation in brightly lit environments is that component of the downwardly incident light which is transmitted through the translucent screen and reflected back to it by the internal mirror.
It would, of course, be possible to avoid retro-reflection of the transmitted component of ambient light be eliminating beam-folding mirrors, and projecting the image directly upon the rear surface of the screen in a straight line path. In order to obtain sufficient path length for adequate magnification, however, this approach would require the use of a housing extending far behind the projection screen. The purpose of using mirrors to fold the image projection beam is to permit the optical system to fit into a housing of reasonably small size and compact shape.